1. Field of the Invention
This invention relates generally to a method of patterning magnetic random access memory (MRAM) cells without sidewall damage using two-step etching & trimming processing.
2. Description of the Related Art
In recent years, magnetic random access memories (hereinafter referred to as MRAMs) using the magnetoresistive effect of a ferromagnetic tunnel junction, also called magnetic tunnel junction (MTJ), have been drawing increasing attention as the next-generation solid-state nonvolatile memories that can cope with high-speed reading and writing, large capacities, and low-power-consumption operations. A ferromagnetic tunnel junction has a three-layer stack structure formed by stacking a recording layer having a changeable magnetization direction, an insulating spacing layer, and a fixed layer that is located on the opposite side from the recording layer and maintains a predetermined magnetization direction.
To record information in such magnetoresistive elements, there has been suggested a write method using spin momentum transfers or spin torque transfer (STT) switching technique, or the so-called STT-MRAM. Depending on the direction of magnetic polarization, STT-MRAM is further clarified as in-plane STT-MRAM and perpendicular pSTT-MRAM, among which pSTT-MRAM is preferred. According to this method, the magnetization direction of a recording layer is reversed by applying a spin-polarized current to a magnetoresistive element. Furthermore, as the volume of the magnetic layer forming the recording layer is reduced, the injected spin-polarized current to write or switch can be also smaller. Accordingly, this method is expected to be a write method that can achieve both device miniaturization and currents reduction.
Patterning a MTJ cell, in a prior art, is by reactive ion etching using CH3OH or CO & NH3 as etchant gases (see U.S. Pat. No. 7,060,194 and U.S. Pat. No. 7,936,027). Very good junction profile (almost or virtually vertical) can be achieved and the etching process is pretty fast. However, there is a damaged layer surrounding a MTJ cell due to high energy ion beam of bombardment. The magnetic property is severely worsened while still electrically conducting, which results in a lower TMR value.
For avoiding such sidewall damages on a MTJ, an alternative way of neutral beam etching was introduced (see U.S. Pat. No. 8,722,543 and ECS Transactions, 35 (4)701-716 (2011) 10.1149/1.3572314 © The Electrochemical Society). However, the entire process is very slow.
Recently developed oxygen neutral beam process technology has attracted attention as a way of solving sidewall damages on a MTJ. A neutral beam suppresses the incidence of charged particles and UV photon radiation onto a device in process, and is able to expose the device only to energy controlled neutral beam (neutral beam motion energy can be precisely controlled by ion acceleration energy with the applied electric field before neutralization), resulting in ultra-precise nanoprocessing that can suppress the formation of defects at the atomic layer level and control surface chemical reactions with high precision. Since the energized particles that inhibit the metal complex reactions, such as ultraviolet rays and electrons are completely cut off, the metal complex reaction is possible even at room temperature in a neutral beam process. As a result, the etching shape by oxidation and complex reaction of the transition metal Ta, Ru, or a Pt becomes ideal shape to the mask as the deterioration of the magnetic properties of the magnetic body which is observed in the plasma etching in the magnetic properties and that was also found that it is possible to suppress completely.
Device using this technology can be implemented in conventional plasma sources, simply by adding a graphite grit for neutralization. It is that the future, and to promote a great deal of device development practical in advancing the research and development of surface modification-modification techniques that does not stop the processing technology in the MRAM manufacturing, using a neutral particle beam.
The above development is found in a publication by X. Gu, Y. Kikuchi, T. Nozawa and S. Samukawa, “A Novel Metallic Complex Reaction Etching for Transition Metal and Magnetic Material by Low-temperature and Damage-free Neutral Beam Process for Non-volatile MRAM Device Applications”, VLSI Technology Symposium, Session 6 Process Technology I, No. 6.5 Jun. 10, 2014 and in US Patent Application No. 2014/0291288.